In the oilfield service industry and specifically in the division known as the wellhead isolation tool service industry, there are several wellhead isolation tools as for example described in: Bullen, A Well Tree Saver, Canadian Patent No. 1,094,945, U.S. Pat. No. 4,241,786, McLeod, an Insertion Drive for Tree Savers, Canadian Patent No. 1,222,204, U.S. Pat. No. 4,632,183, Dallas-Garner, Wellhead Isolation Tool and Setting Device and Method of Using Same, Canadian Patent No. 1,267,078, U.S. Pat. No. 4,867,243. The purpose of these tools is to insert a mandrel with a sealing nipple through a wellhead and into the well casing or tubing where the sealing nipple seals against the inside of the casing or tubing in order to allow high pressure fluid to be injected into the casing or tubing and bypass the wellhead configuration. For this discussion and throughout this patent document, we will refer only to the casing although the same testing apparatus and method will apply to the tubing. Two of the many nipple sealing means as mentioned in Bullen are: McLeod, a Nipple Insert, Canada Patent No. 1,169,766, U.S. Pat. No. 4,601,494, which features a bonded seal and Sutherland-Wenger, a Wellhead Isolation Tool Nipple, Canada Patent No. 1,272,684 which uses a removable seal. It is sometimes found that this sealing nipple on the end of the mandrel in contact with the casing may begin to leak between the casing and the sealing nipple seal under the high pressures encountered after the commencement of a well treatment. This leakage presents a dangerous situation and requires stopping the high pressure treatment of the well and replacing the sealing nipple with one of a tighter fit or a more durable seal material. The industry requires a method of testing the sealing capability of the sealing nipple in place in the well casing prior to beginning the well treatment. The pressure test must be made to a small percentage above the maximum pressure that will be encountered during the well treatment.